Synthesis and Study of the Properties of Stereocontrolled Poly(N-isopropylacrylamide) Gel and Its Linear Homopolymer Prepared inthe Presence of a Y(OTf)3 Lewis Acid: Effect of the Composition ofMethanol−Water Mixtures as Synthesis MediaChandra Sekhar Biswas, Niraj Kumar Vishwakarma, Vijay Kumar Patel, Avnish Kumar Mishra,Satyen Saha, and Biswajit Ray*

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India

*S Supporting Information

ABSTRACT: Poly(N-isopropylacrylamide) (PNIPAM) hy-drogels and the corresponding linear homopolymers weresynthesized in different methanol−water mixtures (xm = 0,0.13, 0.21, 0.31, 0.43, 0.57, and 0.76, where xm is the molefraction of methanol) in the presence of 0.1 M Y(OTf)3 Lewisacid. The isotacticity (meso dyad (m), %) and cloud-pointtemperature of these homopolymers were gradually increasedand decreased, respectively, with the increase in the xm valuesof the synthesis solvent mixtures. Moreover, the correspondinglinear PNIPAM homopolymers prepared in the absence ofY(OTf)3 showed an almost constant isotacticity of m = 45% and a cloud-point temperature of 33.0 °C. A SEM study revealedthat the resulting hydrogels were highly porous except for the gels prepared at xm = 0 and 0.76. The swelling ratios of thesehydrogels in water at different temperatures and in different methanol−water mixtures at 20 °C and the deswelling rate and thereswelling rate of these hydrogels were studied. All of these swelling results were compared with that of the corresponding gelsprepared in the absence of a Lewis acid (Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Bhimireddi, R.; Rai, R.;Ray, B. J. Appl. Polym. Sci. 2012, DOI: 10.1002/app.36318) and explained on the basis of the porosity of the gel, the state ofaggregation and isotacticity of the PNIPAM chain segment, and the cononsolvency of the methanol−water mixture toward thePNIPAM chain segment.

1. INTRODUCTIONPoly(N-isopropylacrylamide) (PNIPAM) homopolymer and itscross-linked gel generally show a volume phase transition inwater at its lower critical solution temperature (LCST) ofaround 33 °C.1 PNIPAM also shows a volume phase transitionin the specific composition of water and its water-miscible goodorganic solvents such as methanol,2−4 ethanol,5 tetrahydrofuran(THF),6,7 dimethyl sulfoxide,7,8 and N,N-dimethylformamide(DMF).8,9 This is essentially due to the cononsolvencyphenomenon10−14 wherein mixtures of two good solventsbecome a poor solvent. Recently, Okamoto et al. reported thesynthesis of isotacticity-rich PNIPAM homopolymers inmethanol15 or a methanol−toluene (1:1 v/v) mixture16−18 inthe presence of rare earth Lewis acids such as Y(OTf)3,Yb(OTf)3, and Sc(OTf)3. They showed that the isotacticity ofthe resultant polymers increased with the increase in theconcentration of the Lewis acid loading, and the solubility ofsuch PNIPAMs in water decreased with the increase in theisotacticity of the polymers. They also showed that the volumephase transition temperature of the resultant polymer decreaseswith increases in its isotacticity.18 Later, Hietala et al. reportedin detail the thermal association properties of A-B-A-typestereoblock PNIPAM copolymers in water.19,20 Very recently,

Nakano et al. reported the thermoreversible gelation of isotactic(m = 64%) PNIPAM in water.21 Recently, we reported thesynthesis and study of the swelling properties and morphologyof the isotactic-rich PNIPAM gels prepared in the presence ofdifferent concentrations of Lewis acid Y(OTf)3 in a 1:1 v/vmethanol−water mixture.22 We also showed there that thevariation in the isotacticity and volume phase transition (cloud-point) temperature of the corresponding PNIPAM homopol-ymers with the Lewis acid loading followed the same trend asreported earlier (Figure S1, Supporting Information).18 Veryrecently, we have also reported the synthesis and study of theswelling properties and morphology of PNIPAM hydrogelsprepared in different ethanol−water mixtures23 and methanol−water mixtures.24 Here, we report the synthesis of a series ofstereocontrolled PNIPAM gels in different compositions ofmethanol−water mixtures in the presence of 0.1 M Y(OTf)3and the study of their morphology, swelling ratios in water atdifferent temperatures and in different methanol−watermixtures at 20 °C, deswelling kinetics in water when swiftly

Received: January 26, 2012Revised: April 4, 2012Published: April 4, 2012

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changing the temperature from 20 to 40 °C, and reswellingkinetics in water at 20 °C. To understand the effect of thesolvent composition in the presence of the Lewis acid on theproperties of the PNIPAM chain segment in the formed gel, wehave also synthesized the corresponding linear PNIPAMhomopolymers in the absence and presence of 0.1 M Y(OTf)3and determined their tacticities and cloud-point temperatures.We have also compared these results with the reported resultsobtained from the similar gel systems prepared in the absenceof the Lewis acid.24

2. EXPERIMENTAL SECTION2.1. Materials. N-Isopropylacrylamide (NIPAM, Aldrich, St. Louis,

MO) was purified by recrystallization from n-hexane. N, N′-Methylenebisacrylamide (BIS, Aldrich, St. Louis, MO), ammoniumpersulfate (APS, Loba Chemie, Mumbai, India), N,N,N′,N′-tetrame-thylethylenediamine (TEMED, Aldrich, St. Louis, MO), and yttriumtrifluoromethanesulfonate [Y(OTf)3, Aldrich, St. Louis, MO] wereused as received. Methanol (Loba Chemie, Mumbai, India) was driedand distilled over anhydrous calcium oxide. Deionized water wasprepared by the redistillation of doubly distilled water in an all-glassdistillation apparatus.2.2. Synthesis of PNIPAM Hydrogels. Three stock solutions

were prepared: (i) a solution of TEMED in water having aconcentration of 107 mmol/dm3; (ii) a solution of TEMED inmethanol having a concentration of 107 mmol/dm3; and (iii) asolution of APS in water having a concentration of 84 mmol/dm3. Atfirst, the required amounts (as specified in Table 1) of NIPAM,Y(OTf)3, BIS, and TEMED solutions and solvents were taken in asmall borosilicate glass tube (i.d. = 6 mm, length = 100 mm) fitted

with a rubber septum. The rest of the procedure is the same asreported earlier.22

2.3. Synthesis of Linear PNIPAM Homopolymers. A series oflinear PNIPAM homopolymers (runs X0′−X0.76′, Table 2) weresynthesized in the absence of the BIS cross-linker by keeping otherexperimental conditions the same as mentioned in the synthesis ofPNIPAM hydrogels (runs X0−X0.76, Table 1). The polymerization andpurification procedures are the same as reported earlier.22 Anotherseries of linear PNIPAM homopolymers (runs X0′′−X0.76′′, Table S1,Supporting Information) were synthesized in the absence of the BIScross-linker and Lewis acid Y(OTf)3 by keeping other experimentalconditions the same as mentioned in the synthesis of PNIPAMhydrogels (runs X0−X0.76, Table 1).

1H NMR spectra of the resulting polymers were recorded at 130 °Con a JEOL AL300 FTNMR (300 MHz) in DMSO-d6 solvent and werereported in parts per million (δ) from residual solvent peaks. The diadtacticities of polymer samples were calculated from the methyleneproton peaks of the polymers as reported earlier in the literature.22

Cloud-point temperature of the polymer was determined using a Cary100Bio UV−vis spectrophotometer (Varian) equipped with a Peltierseries II thermostatic cell holder by the method reported earlier.22

2.4. Surface Morphology. Equilibrium-swollen hydrogels indeionized water at 20 °C for 24 h were freeze-dried under vacuumto remove water completely. FEI-SEM Quanta 200F (Philips) at anacceleration voltage of 5 kV was used to study the surface morphologyof these freeze-dried samples.

2.5. Swelling Ratios in Water at Different Temperatures. Thepreweighed dried gels were immersed in deionized water for 24 h atthe desired temperature (20, 22.5, 27.5, 30, 32.5, 35, 38, and 40 °C).Then, these equilibrium-swollen gels were taken out, the surface waterwas soaked with moistened filter paper, and their weights were taken.

Table 1. Synthesis of Poly(N-isopropylacrylamide) Gels in the Presence of a 0.1 M Y(OTf)3 Lewis Acid in DifferentCompositions of Methanol−Water Mixturesa

run ID

X0 X0.13 X0.21 X0.31 X0.43 X0.57 X0.76

MeOH (mL) 0.5 0.75 1.0 1.25 1.0 1.25water (mL) 1.25 0.75 0.5 0.25 0.25solution of TEMED (107 mmol/dm3) in water (mL) 0.5 0.5 0.5 0.5 0.5solution of TEMED (107 mmol/dm3) in methanol (mL) 0.5 0.5conversion (%)b 89 93 91 89 86 92 96appearence transparent opaque opaque opaque opaque transparent transparentswelling ratio (Ws/Wd) at 20 °Cc 11.9 18.3 25.7 15.5 26.5 17.0 5.4swelling ratio (Ws/Wd) at 40 °Cc 2.0 1.7 1.7 1.5 1.5 1.4 1.7

aNIPAM = 160 mg; BIS = 8 mg; Y(OTf)3 = 108 mg; APS = 0.25 mL of an aqueous solution with a concentration of 84 mmol/dm. Polymerizationtemperature = 5 °C, polymerization time = 12 h. bDetermined gravimmetrically after drying under vacuum at 50 °C for 72 h after dialysis. cWs =weight of the swelled gel at a specified temperature after 24 h of swelling, Wd = weight of the dry gel.

Table 2. Synthesis of Poly(N-isopropylacrylamide) Homopolymer in the Presence of a 0.1 M Y(OTf)3 Lewis Acid in DifferentCompositions of Methanol−Water Mixturesa

run ID

X0′ X0.13′ X0.21′ X0.31′ X0.43′ X0.57′ X0.76′

MeOH (mL) 0.5 0.75 1.0 1.25 1.0 1.25water (mL) 1.25 0.75 0.5 0.25 0.25solution of TEMED (107 mmol/dm3) in water (mL) 0.5 0.5 0.5 0.5 0.5solution of TEMED (107 mmol/dm3) in methanol (mL) 0.5 0.5conversion (%)b 90 94 92 95 97 91 97appearence transparent opaque opaque opaque opaque transparent transparenttacticityc 45 50 53 55 62 71 81cloud pointd (°C) 33.3 31.5 30.7 30.3 28 e e

aNIP AM = 160 mg, Y(OTf)3 = 108 mg, APS = 0.25 mL solution in water of 84 mmol/dm3 concentration, polymerization temperature = 5 °C,polymerization time = 12 h. bDetermined gravimmetrically after drying under vacuum at 50 °C for 72 h after dialysis. cDetermined by 1H NMR inDMSO-d6 at 130 °C. dDetermined by measuring the transmittance of 500 nm light through a 1% (w/v) aqueous polymer solution with a 0.5 °C/min heating and cooling rate. eInsoluble in water.

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The swelling ratio (Ws/Wd) was calculated as the ratio of the weight ofthe equilibrium-swollen gel (Ws) to that of the dried gel (Wd).2.6. Swelling Ratios in Different Methanol−Water Mixtures

at 20 °C. The swelling ratios of the different gels in the methanol−water mixtures containing 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4,0.45, 0.5, 0.55, 0.6, 0.8, and 1.0 mol fractions of methanol (xm) at 20°C were measured gravimetrically using the same method as describedabove.2.7. Deswelling Kinetics in Water at 40 °C. The preweighed

equilibrium-swollen gels in water at 20 °C for 24 h were immersedquickly in water at 40 °C. At definite time intervals, the gel was takenout, the surface water was soaked with moistened filter paper and itsweight was taken, and then the gel was quickly re-immersed in thewater at 40 °C. Water retention (%) was calculated as the weightpercentage of the water retained (Wt −Wd) by the swollen gel (Wt) atany definite time interval (t) with respect to that (Ws − Wd) by theequilibrium-swollen hydrogel (Ws) at 20 °C.2.8. Reswelling Kinetics in Water at 20 °C. The preweighed

equilibrium-swollen gels at 40 °C for 24 h were immersed quickly inthe water at 20 °C. At definite time intervals, the gel was taken out, thesurface water was soaked with moistened filter paper and its weightwas taken, and then the gel was quickly re-immersed in the water at 20°C. The water uptake (%) was calculated as the weight percentage ofwater absorbed by the swollen hydrogel at any definite time interval t(Wt − Wd) with respect to that by the equilibrium-swollen hydrogel(Ws − Wd) at 20 °C.

3. RESULTS AND DISCUSSION

3.1. Synthesis of PNIPAM Hydrogels in DifferentMethanol−Water Mixtures in the Presence of a 0.1 MY(OTf)3 Lewis Acid. The synthesis conditions and thecharacterization of PNIPAM hydrogels are summarized inTable 1.In runs X0−X0.76, the methanol−water mixtures with xm

values of 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76 were used assynthesis media in the presence of a 0.1 M Y(OTf)3 Lewis acid.Yields of the PNIPAM gels were within 86−96%. Theappearances of the as-prepared hydrogels changed fromtransparent (run X0) to opaque (runs X0.13, X0.21, X0.31, andX0.43) and then changed back to transparent (runs X0.57 andX0.76). The observed transparency of the hydrogels prepared atxm = 0 (run X0), 0.57 (run X0.57), and 0.76 (run X0.76) was dueto the highly solvated coiled conformation of the PNIPAMchain segment in these gels owing to the stronger interaction ofwater or such methanol−water mixtures with the correspond-ing PNIPAM chain segment. However, the observed opacity ofthe gels prepared at xm = 0.13 (run X0.13), 0.21 (run X0.21), 0.31(run X0.31), and 0.43 (run X0.43) was indicative of the formationof the less-solvated aggregated globular PNIPAM chainsegment due to the cononsolvency of the correspondingmethanol−water mixtures toward the PNIPAM chain segmentin the gels.To obtain an idea of the effect of the methanol−water

composition in the presence of the 0.1 M Y(OTf)3 Lewis acidon the tacticity of the PNIPAM chain segment in thesynthesized gels, we have prepared a series of correspondinglinear PNIPAM homopolymers (runs X0′−X0.76′, Table 2) in theabsence of the BIS cross-linker while keeping otherexperimental conditions the same as mentioned for runs X0−X0.76 in Table 1. The appearance of the as-preparedhomopolymerization mixtures (runs X0′−X0.76′) was similar tothat of the corresponding hydrogels (runs X0−X0.76, respec-tively), as discussed above. Polymer yields were within 90−97%. 1H NMR spectra of PNIPAM homopolymers prepared inruns X0′−X0.76′ (Table 2) are shown in Figures S2 and S3

(Supporting Information), and the corresponding plot of theobserved isotacticities (meso dyad (m), %) of the resultingpolymers against the corresponding xm values is shown inFigure 1.

The isotacticity (m) of the resulting polymers increasedgradually from 45 to 81% with the gradual increase in the xmvalues from 0 to 0.76, respectively. Initially, it slowly increasedfrom 45 to 55% with the increase in the xm values from 0 to0.31 (runs X0−X0.31), respectively. Then, it increasedsignificantly from 55 to 81% with further increases in the xmvalue from 0.31 to 0.76 (runs X0.31−X0.76), respectively. It is tobe noted here that the isotacticity of the PNIPAMhomopolymer prepared in water in the absence of the Lewisacid is 43%.22 These results clearly indicate the roles of both theLewis acid and the xm values of the polymerization mediumwith respect to the tacticity of the PNIPAM chain segment inthe resulting polymers. The presence of the Lewis acid isnecessary to increase the isotacticity, as reported earlier.15−22

Here, for a specific concentration of Lewis acid, the higher thexm value of the synthesis solvent, the higher the isotacticity ofthe PNIPAM chain segment. In this regard, the observed slowincrease in tacticity of PNIPAMs in close proximity to thecononsolvency zone (xm = 0.13, 0.21, and 0.31) may have beendue to a small increase in the interaction of Y(OTf)3 with theamide groups of the active propagating PNIPAM chain-endradical species and the incoming NIPAM monomers owing tothe cononsolvency of such methanol−water mixtures. Thisagain clearly confirms the role of methanol in producingisotactic PNIPAM in the presence of the Y(OTf)3 Lewisacid.15−21 Therefore, the higher the xm value of the synthesissolvent, the higher the isotacticity of the PNIPAM chainsegment in the formed gel. Moreover, the higher the isotacticityof the PNIPAM chain segment in the formed gel, the lower thehydrophilicity of the PNIPAM chain segment in the gel.22 Theobserved cloud-point temperature of these linear PNIPAMhomopolymers gradually decreased from 33.3 to 28 °C with theincrease in xm values of the polymerization media from 0 to0.43, respectively, due to the gradual increase in the tacticity(Figure 1) (vide Figure S4 (Supporting Information) and Table

Figure 1. Plot of the mole fraction of methanol (xm) of the synthesissolvent vs the tacticity (meso %) and cloud point (°C) of thehomopolymer and the relative change in the swelling ratio in water at20 °C (%) of the gel prepared in the presence of the Lewis acid withrespect to that of the same prepared in the absence of the Lewis acid.

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2). Similar types of results were also reported earlier.18,22 Thedetermination of the cloud-point temperature for the linearPNIPAM homopolymers prepared with xm = 0.57 and 0.76(runs X0.57′ and X0.76′, respectively) was not possible because oftheir insolubility in water owing to their very high isotacticity.To get an idea of the effect of only the methanol−water

composition (xm) on the tacticity of the PNIPAM chainsegment in the formed gel, we have also prepared a series oflinear PNIPAM homopolymers (runs X0″−X0.76″, Table S1(Supporting Informations)) in the absence of the BIS cross-linker and the Y(OTf)3 Lewis acid by keeping otherexperimental conditions the same as mentioned for runs X0−X0.76 in Table 1. The appearance of the as-preparedhomopolymerization mixtures (runs X0″−X0.76″) was similarto that of the corresponding hydrogels (runs X0−X0.76,respectively) as discussed above. Polymer yields were within81−93%. The isotacticities (m) of these resulting polymers areclose to ∼45% (vide Table S1 (Supporting Information),Figure 1). Moreover, the observed cloud-point temperatures ofthese polymers are close to 33.0 °C (vide Figure 1, Table S1and Figure S5 (Supporting Information)). These results clearlyindicate the almost negligible effect of the xm value of thepolymerization media on the isotacticity and cloud-pointtemperature of the formed polymers in the absence of theLewis acid.3.2. Surface Morphology. The SEM images (magnifica-

tion ×6000) of all of the freeze-dried hydrogels are shown inFigure 2. The gels prepared at xm = 0.76 (Figure 2g, run X0.76)are macroscopically homogeneous, as evident from the absenceof any apparently visible pores. This is due to the strongerinteraction of PNIPAM chain in the gel with the correspondingsynthesis solvent mixture. It is to be noted here that methanolor methanol-rich methanol−water mixtures are good solventsfor isotactic (m)-rich PNIPAM chain segments.17,22 However,the gels prepared at other xm values of the synthesis solvent(runs X0.13−X0.57) have macroporous morphology (Figure 2b−f, respectively). This is due to (i) the increase in thepolymerization rate in the presence of methanol and Lewisacid Y(OTf)3 owing to the faster decomposition of theammonium persulfate initiator25 and (ii) the decrease in thesolvency of the PNIPAM chain segment owing to its isotacticityand the cononsolvency of the synthesis medium. Moreover, the

pore sizes of the gels are maximized in the cononsolvency zone(at xm = 0.21, 0.31, and 0.43; runs X0.21, X0.31, and X0.43,respectively). A similar trend was also observed for PNIPAMgels prepared in different methanol−water mixtures in theabsence of the Lewis acid.24 The gel prepared in water (xm = 0)(Figure 2a, run X0) has very small pores owing to the relativelyslower polymerization rate in the absence of methanol.

3.3. Swelling Ratios in Water at Different Temper-atures. The swelling ratios (Ws/Wd) in water at differenttemperatures within 20−40 °C for all of the hydrogels (runsX0−X0.76, Table 1) are shown in Figure 3. The swelling ratio

values in water at 20 °C of all of these gels are separately shownin Figure 4. Here, the equilibrium swelling ratio of these gels inwater at 20 °C varied in the following order: X0.43 ≈ X0.21 >X0.13 > X0.57 > X0.31 > X0 > X0.76 (Table 1 and Figures 3 and 4).Similar order was also observed for the gels prepared in theabsence of the Lewis acid except for the gel prepared at xm =0.76.24 The observed very low swelling ratio (5.4) for the gelprepared in run X0.76 (at xm = 0.76) was due to the formation of

Figure 2. SEM images of the hydrogels synthesized in the presence of the 0.1 M Y(OTf)3 Lewis acid in methanol−water mixtures with xm values of(a) 0 (X0), (b) 0.13 (X0.13), (c) 0.21 (X0.21), (d) 0.31(X0.31), (e) 0.43 (X0.43), (f) 0.57 (X0.57), and (g) 0.76 (X0.76).

Figure 3. Equilibrium swelling ratios of all of the PNIPAM hydrogels(runs X0−X0.76, Table 1) in water at 20, 22.5, 27.5, 30, 32.5, 35, 38, and40 °C.

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the gel with an invisible pore size and a very highly isotactic (m= 81%) PNIPAM chain segment. Interestingly, the correspond-ing relative change (%) in the swelling ratio in water at 20 °C ofthese gels prepared in the presence of Y(OTf)3 with respect tothat of the gel prepared in the absence of the Lewis acid24

initially increases, decreases slowly, passes through an almostflat region in the cononsolvency zone, and finally decreasessteeply with the increase in the xm value of the synthesis media(Figure 1). Initial observed increments (+ve) in the relativeswelling ratio at xm = 0 and 0.13 are due to the predominanteffect of the porosity (vide Figure 2a,b) over the isotacticity.Then, it follows that there is a slight decrease (becomesnegative (−ve)) at xm = 0.21 due to the predominant effect ofisotacticity over porosity. The observed almost slow decrease inthe cononsolvency zone might be due to the combined effectsof the increases in isotacticity (and insolubility) of thePNIPAM chains and the porosity of the formed gels. Beyondthe cononsolvency zone, it decreases sharply because ofpredominant effects of the increment of isotacticity. It is tobe mentioned here that the relative change (%) in the swellingratios in water at 20 °C of the cross-linked PNIPAM gelsprepared in a 1:1 v/v methanol−water mixture (xm = 0.31) inthe presence of different Y(OTf)3 concentrations22 initiallyincreases because of the formation of a highly porous gel owing

to the faster rate of polymerization (cross-linking) (Figure S1,Supporting Information). Here, although the tacticity is slightlyincreased in the presence of the Lewis acid, the porosity factoralso dominates the tacticity factor. Then, it decreasesexponentially because of the exponential increase in thetacticity of the PNIPAM chain segment in the formed cross-linked gel.The swelling ratio (Ws/Wd) in water at 40 °C for all of the

hydrogels was close to 2 (Table 1 and Figure 3). This was dueto the complete collapse of the coiled conformation of thePNIPAM chain segment into its slightly solvated globular format this temperature.In general, below the LCST, the swelling ratio values

gradually decreased with the increase in the temperaturebecause of the release of water due to the gradual collapse ofthe PNIPAM chain segment in the gel (Figure 3). Similar typesof results have also been reported for PNIPAM gels in theliterature for mixtures of water and different water-misciblesolvent systems.6,9,26−31

The comparative results of the swelling ratio in water atdifferent temperatures of the gels prepared in differentmethanol−water mixtures in the absence24 and presence of0.1 M Lewis acid are shown in Figure 5. The swelling ratios ofthe gels prepared at xm = 0 and 0.13 in the presence of theLewis acid are higher mainly because of the predominant effectof the porosity over the tacticity. Apart from this, in general,swelling ratios of the gels prepared at higher xm values in thepresence of the Lewis acid are lower because of the gradualincrease in the isotacticity. More or less, these values arecomparable for all of the gels prepared in the cononsolvencyzone (xm = 0.21, 0.31, and 0.43). This clearly indicates thepredominant effect of the cononsolvency of methanol−watermixtures. Above xm = 0.43, these values deviate gradually fromeach other mainly because of the increase in the isotacticity ofthe PNIPAM chain and are maximized at xm = 0.76.

3.4. Swelling Ratios in Different Methanol−WaterMixtures at 20 °C. The changes in the swelling ratio (Ws/Wd)of all PNIPAM gels in different methanol−water mixtures (xm)at 20 °C are shown in Figure 6.The swelling ratios of all of the hydrogels in different

methanol−water mixtures at 20 °C passed through a minimumin the cononsolvency zone. Such an observation was also madefor PNIPAM hydrogels prepared in the absence of the Lewisacid by keeping the other conditions the same.24 Interestingly,the PNIPAM hydrogel prepared at xm = 0.76 (run X0.76)showed a very early onset of cononsolvency at xm = 0.05, and

Figure 4. Plot of the swelling ratio of the hydrogels in water at 20 °Cagainst the mole fraction of methanol (xm) in the synthesis solvent inthe presence and absence (ref 24) of Lewis acid Y(OTf)3.

Figure 5. Comparative results of the swelling ratio in water at different temperatures for the gels prepared in different methanol−water mixtures inthe absence (ref 24) and presence of 0.1 M Lewis acid.

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the minimum swelling ratio (∼2) was observed at xm = 0.1.This is due to the very poor interaction between its highlyisotactic PNIPAM chain segment and such water-richmethanol−water mixtures; consequently, the solvent uptaketendency decreased. Then, the swelling ratio of this gel (runX0.76) increased gradually with the increase in the methanolcontent, as in other gels. However, unlike the other gels, theobserved swelling ratio was much higher in pure methanol thanin water. This was due to the greater solvency of the highlyisotactic PNIPAM chain segment of the gel in methanol.The comparative results of the swelling ratio of the gels

prepared in the absence24 and presence of 0.1 M Lewis acid areshown in Figure 7. It is clear from the figure that swelling ratiosof all of the gels for both systems are comparable, except for thegels prepared at xm = 0.57 and 0.76. The observed discrepanciesfor xm = 0.57 and 0.76 gel systems (runs X0.57 and X0.76) aremainly due to the higher isotacticity of the PNIPAM chainsegment in the formed gel.3.5. Deswelling Kinetics in Water at 40 °C. The time-

dependent water retention (%) of all of the hydrogels when thetemperature was increased instantly from 20 to 40 °C is shownin Figure 8. The deswelling rate of these hydrogels decreased in

the following order: X0.43 > X0.31 > X0.21 > X0.57 > X0.13 > X0.76 >X0.Here, this rate expectedly depends on the isotacticity of the

PNIPAM chain segment apart from other factors such as theporosity, state of aggregation of the PNIPAM chain segment,and so forth. The greater all of these factors, the faster thedeswelling of the gels. The deswelling rate was observed to beslowest with the hydrogel prepared at xm = 0 (run X0). Theobserved, considerably faster deswelling rate for the gelprepared at xm = 0.13 (run X0.13) was due to its higherporosity (Figure 2b), relatively more aggregated PNIPAMchains owing to the onset of cononsolvency, and more highlyisotactic PNIPAM chains. This rate increased further for runX0.21 because of the further increment in these factors at xm =0.21 (vide Figures 1 and 2c). It increased further gradually butslowly for gels prepared at xm = 0.31 (run X0.31) and 0.43 (runX0.43) presumably because of the increase in the isotacticity.However, the observed slower rate of the gel prepared at xm =0.57 (run X0.57) may possibly be due to the formation ofmacroporous gel morphology (Figure 2f) containing lessaggregated and more highly isotactic (m = 71%) PNIPAMchain segments. The observed further slower deswelling rate ofthe gel prepared at xm = 0.76 (run X0.76) was due to theformation of a macroscopically homogeneous gel (Figure 2g)

Figure 6. Equilibrium swelling ratios of all of the PNIPAM hydrogels(runs X0−X0.76, Table 1) in methanol−water mixtures with xm valuesof 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.8, and1.0 at 20 °C.

Figure 7. Comparative results of the swelling ratios in different methanol−water mixtures at 20 °C for all of the gels prepared in different methanol−water mixtures in the absence (ref 24) and presence of 0.1 M Lewis acid.

Figure 8. Deswelling kinetics of the PNIPAM hydrogels synthesized inthe presence of 0.1 M Y(OTf)3 Lewis acid in methanol−watermixtures with xm values of 0 (run X0), 0.13 (run X0.13), 0.21 (runX0.21), 0.31 (run X0.31), 0.43 (run X0.43), 0.57 (run X0.57), and 0.76 (runX0.76).

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containing relatively less aggregated and more highly isotacticPNIPAM chain segments (Figure 1).The comparative results of the deswelling kinetics of the gels

prepared in the absence24 and presence of 0.1 M Lewis acid areshown in Figure 9. It is clear from the figure that the deswellingrates of all of the gels prepared in the presence of the Lewis acidare faster with respect to those prepared in the absence of theLewis acid. Interestingly, this rate is very fast and comparablefor the gels prepared in the cononsolvency zone (xm = 0.21−0.43). This result indicates that this rate is almost independentof the isotacticity of the PNIPAM chain in the gels prepared inthis zone. However, for the gels prepared in the precononsol-vency zone (xm = 0 and 0.13), the porosity factor mainlyinduced such a faster rate. However, the isotacticity of thePNIPAM chain of the gels predominantly induced such a fasterrate for the gels prepared in the postcononsolvency zone (xm =0.57 and 0.76).3.6. Reswelling Kinetics in Water at 20 °C. The

reswelling rate of all of the hydrogels at 20 °C in water, aftershrinking at 40 °C for 24 h, is shown in Figure 10. This ratedecreased in the following order: X0 > X0.31 > X0.76 > X0.13 >X0.57 > X0.21 > X0.43. Here, this rate expectedly depends on theporosity, the state of aggregation and isotacticity of the

PNIPAM chain segment in gel matrix, and so forth. It increaseswith increase in porosity and decrease in both the state ofaggregation and isotacticity of the PNIPAM chain segment.Therefore, the observed fastest rate for run X0 is mainly due toits less aggregated and lowest isotacticity PNIPAM chain(Figure 1). The observed gradual significant decrease in the ratefrom runs X0 through X0.13 to X0.21 is due to the predominanceof the increase in the state of aggregation and the isotacticity ofthe PNIPAM chain segment (Figure 1) over the porosity of thegel (Figure 2a−c, respectively). The observed unusual fasterreswelling rate for run X0.31 may have been due to itsmacroporous morphology with a highly cross-linked networkstructure (Figure 2d). Such behavior was also observed for thePNIPAM gel prepared in the absence of the Lewis acid.24 Theobserved slowest reswelling rate for run X0.43 is due to thepredominance of the higher isotacticity and state of aggregationof its PNIAM chain segment over the porosity factor. A slightincrease in the reswelling rate for run X0.57 is presumably due tothe predominance of the decrease in the state of aggregation ofthe PNIPAM chain owing to its higher solvency over thetacticity and porosity factors. The observed faster reswellingrate for run X0.76 is also due to the same reasons as for run X0.57.It is to be mentioned here that the observed swelling ratio ofthis gel is very low (vide Figure 4) because of the very highisotacticity of the PNIPAM chain segment and the macro-scopically homogeneous surface morphology.The comparative reswelling rate in water at 20 °C of the gels

prepared in different methanol−water mixtures in the absence24and presence of 0.1 M Lewis acid is shown in Figure 11. It isclear from the figure that the reswelling rates are comparablefor all gels and therefore are almost independent of theisotacticity of the PNIPAM chain.

4. CONCLUSIONSPNIPAM hydrogels and the corresponding homopolymerswere synthesized in different methanol−water mixtures (xm =0−0.76) in the presence of 0.1 M Y(OTf)3 Lewis acid. Theobserved isotacticity (m, %) and the cloud-point temperature ofthe resulting homopolymers gradually increased and decreased,respectively, with the increase in the xm value of the synthesissolvent. The corresponding linear PNIPAM homopolymersprepared in the absence of Y(OTf)3 showed an almost constantisotacticity (m, %) (45%) and cloud-point temperature (33°C). The SEM study revealed that the resulting hydrogels werehighly porous except for the gels prepared at xm = 0 and 0.76.The swelling ratios of all of the hydrogels (prepared in thepresence and absence of 0.1 M Lewis acid) in water decreased

Figure 9. Comparative results of the deswelling kinetic in water at 40 °C of the gels prepared in different methanol−water mixtures in the absence(ref 24) and presence of 0.1 M Lewis acid.

Figure 10. Reswelling kinetics of the PNIPAM hydrogels synthesizedin the presence of 0.1 M Y(OTf)3 Lewis acid in methanol−watermixtures with xm values of 0 (run X0), 0.13 (run X0.13), 0.21 (runX0.21), 0.31 (run X0.31), 0.43 (run X0.43), 0.57 (run X0.57), and 0.76 (runX0.76).

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with the increase in temperature. The swelling ratios of all ofthe gels (prepared in the presence and absence of 0.1 M Lewisacid) in different methanol−water mixtures at 20 °C passedthrough a minimum in the cosolvency zone and are observed tobe comparable except for the gels prepared at xm = 0.57 and0.76. The deswelling rates of the hydrogels decreased in thefollowing order: X0.43 > X0.31 > X0.21 > X0.57 > X0.13 > X0.76 > X0.In general, the deswelling rates of these gels are faster withrespect to the rates of those prepared in the absence of theLewis acid. The observed faster rates for the gels prepared inthe precononsolvency (xm = 0 and 0.13), cononsolvency (xm =0.21 to 0.43), and postcononsolvency zones (xm = 0.57 and0.76) are predominantly due to the porosity, cononsolvency,and isotacticity of the PNIPAM chain, respectively. Thereswelling rate of the hydrogels decreased in the order X0 >X0.31 > X0.76 > X0.13 > X0.57 > X0.21 > X0.43, and this rate is almostindependent of the isotacticity of the PNIPAM chain.

■ ASSOCIATED CONTENT*S Supporting InformationPlot of the Lewis acid concentration versus the tacticity andcloud point of the PNIPAM homopolymers. Relative change inthe swelling ratio in water at 20 °C of the PNIPAM gels.Synthesis and characterization data of linear PNIPAMhomopolymers in the absence of the Y(OTf)3 Lewis acid.Results of 1H NMR and the cloud-point determination. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: biswajitray2003@yahoo.co.in.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge the financial support from theCouncil of Scientific and Industrial Research, Government ofIndia, through grant no. 01(2099)/07/EMR-II. C.S.B. alsoacknowledges the University Grant Commission for financialsupport. N.K.V., V.K.P., and A.K.M. acknowledge CSIR,Government of India, for research fellowships.

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Figure 11. Comparative results of the reswelling rate in water at 20 °C of the gels prepared in different methanol−water mixtures in the absence (ref24) and presence of 0.1 M Lewis acid.

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